1. Field of the Invention
The invention relates generally to the field of drilling wellbores through subsurface formations to produce hydrocarbons. More specifically, the invention relates to methods for identifying gas productive subsurface formations in such wellbores.
2. Background Art
Wellbores are drilled through subsurface formations to extract hydrocarbons, among other purposes for such wellbores. Drilling a wellbore using processes known in the art includes suspending a “string” of drill pipe or similar tubular apparatus from a structure called a “rig” at the Earth's surface. The string typically includes a drill bit at its lower end. The rig includes equipment to rotate the string while the bit is urged into contact with the subsurface formations. A pump lifts drilling fluid (“mud”) from a pit or tank and moves it through a central passage in the pipe string. The mud is discharged through courses or nozzles in the drill bit to cool the bit, to lubricate the bit, and to lift drill cuttings to the Earth's surface.
Typically the drilling mud includes “weighting material” in suspension such as barium sulfate (“barite”) or similar high density solid particulate material, such that the drilling mud has a density selected to exert a predetermined hydrostatic pressure on the drilled subsurface formations. Such pressure may be the same as or slightly greater than the expected pressure of fluids in the various subsurface formations. Such hydrostatic pressure serves to maintain mechanical stability of the wellbore as it is drilled, and to prevent uncontrolled influx of fluids into the wellbore during drilling.
More recently, it has become known in the art to drill wellbores using drilling mud density slightly below the expected pressure of subsurface formations. Such drilling technique is referred to as “underbalanced drilling” and has as one purpose the reduction or avoidance of damage to the permeability of certain formations by the action of the drilling mud on such formations. It is desirable when drilling such formations to be able to identify those that may be productive of certain hydrocarbons, such as natural gas.
One method for determining the presence of gas entering a wellbore is described in U.S. Pat. No. 5,222,048 issued to Grosso et al. The method described in the '048 patent includes deploying a measurement while drilling (“MWD”) instrument in a wellbore during the drilling thereof. The MWD instrument includes a device for modulating pressure in the drill string so that signals can be communicated to the Earth's surface from the MWD instrument. Such pressure modulation also causes detectable pressure variations in an annular space between the wall of the wellbore and the drill string. Fluid influx into the wellbore is detected by examining and monitoring the computed annulus acoustic signal transfer function. When a downhole energy source, such as MWD pressure pulses, travel to the surface through the drillstring channel and through the annulus channel, the source becomes distorted. The shape (e.g. waveform) of the original signal is known, either because a pre-determined message is sent, or because the transmitted message is reconstructed at the surface. This known transmitted signal is processed with the received signal (which includes the channel distortion) in order to determine the transfer function H(s), of the annulus channel. When a fluid possessing different physical properties as the drilling fluid, such as gas, intrudes into the annular space, the annulus transfer function H(s) will most likely be changed and this change can be used to determined the intrusion. Because the presence of gas in the annulus fluid will greatly change the transfer function H(s) of the annulus signal, this change can be particularly useful as an indicator of gas influx.
U.S. Pat. No. 4,692,908 issued to Ekstrom et al. describes An acoustic method and apparatus for investigating an earth formation penetrated by a borehole are described. Acoustic transducers are mounted on a tool to accurately determine the distance between a segment of the tool and the wall of the borehole. The acoustic transducers are positioned in such manner that the stand-off distance between individual resistivity measuring electrodes in an array, which is also mounted on the tool segment, and the borehole wall can be measured. The stand-off measurement is recorded and may be used to correct the electrode resistivity measurements. In one embodiment a calibration of acoustic transducers as a function of depth is obtained by employing acoustic calibrating transducers to compensate for borehole environment effects on the performance of the acoustic transducers as well as determine the acoustic velocity of the borehole fluid, such as mud. See also U.S. Pat. No. 6,957,700 issued to Mandal.
There continues to be a need for improved methods to determine the presence of gas in a wellbore during and after drilling.